1. Field of the Invention
The present invention relates to an elongate composite insulator which comprises an elongate core rod composed, e.g., of a fiber-reinforced plastics (FRP), a sheath which covers the core rod over substantially the entire length thereof, and a plurality of sheds which are formed on the sheath and spaced from each other in the axial direction of the core rod, wherein the sheath and the sheds are formed by integrally molding of an insulating polymeric material. The present invention also relates to a method for manufacturing such composite insulators.
2. Description of the Related Art
There is known a composite insulator which can be obtained by integrally providing an elongate core rod of fiber-reinforced plastics (FRP) with a sheath and sheds by an injection molding of an appropriate insulating polymeric material such as silicone rubber, ethylene propylene copolymer (EPM), ethylene propylene diene copolymer (EPDM), polyurethane, etc. Such a composite insulator has been actually put into practical applications, particularly in any use environment which can draw out various functional advantages of the composite insulator. As for the injection molding method for manufacturing such composite insulators, various improvements have been conventionally proposed.
For manufacturing relatively short composite insulators of a low withstand voltage class, there has been conventionally adopted a method in which a core rod is placed within a mold cavity of a molding machine and a raw material in the form of an insulating polymeric material, e.g., an unvulcanized rubber, is injected into the mold cavity to integrally form a sheath and sheds on the core rod (see JP-A-60-500929, for example). In the case of a longer composite insulator to which a high withstand voltage is required, however, it is generally difficult to obtain an elongate molded product through a single molding process due to a restriction in length of the mold, for example.
In order to eliminate such a difficulty and allow a simplified and facilitated manufacture of elongate composite insulators with a satisfactory quality, JP-A-6-203680 discloses the applicant's earlier proposal wherein an axially continuous sheath of an elongate composite insulator is formed in a stepwise manner. In this instance, a FRP core rod is placed only partly within a mold cavity of a molding machine, and a first molded portion corresponding to a part of the sheath and sheds is formed on that portion of the core rod which is situated within the mold cavity. The core rod is subsequently displaced in the axial direction by a predetermined amount relative to the mold and a second and succeeding molded portions are formed, which are similar to the first molded portion, with the respective molded portions being simultaneously integrated with each other so that an elongate composite insulator has an axially continuous sheath and associated sheds.
According to the applicant's earlier proposal mentioned above, because an axially continuous sheath having a desired length is formed by integrating a plurality of molded portions which are succeedingly formed on the core rod in axial alignment with each other, it is possible to manufacture an elongate composite insulator having a high accuracy and an excellent insulating property, in a facilitated and simplified manner, without increasing the length of the mold.
As disclosed in JP-A-6-203680, the molding machine capable of manufacturing elongate composite insulators according to the above-mentioned proposal includes an end forming member arranged in a rear end portion of a mold cavity, for forming a rear end of a molded portion which forms an integral part of the sheath. A space for accommodating a front end region of the molded portion is provided adjacent to a front end portion of the metal mold cavity. A first jig for forming a head portion of the sheath, and a second jig for positioning the core rod relative to the mold by clamping the front end of the molded portion can be selectively attached to and removed from the front end of the mold cavity. In this instance, the second jig at the front end of the mold cavity may be composed of a set of arcuate or semi-circular segments which serve to prevent a leakage of the polymeric material which has been injected into the mold cavity.
When an elongate composite insulator is manufactured with the above-mentioned known molding machine, the first jig is attached to the front end portion of the mold cavity and a first molded portion having a head and a rear end is formed on the core rod. The core rod is then displaced in the axial direction and the first jig in the front end of the mold cavity is replaced by the second jig. Subsequently, the second molded portion is formed on the core rod with the rear end of the first molded portion clamped by the second jig and the core rod positioned relative to the mold. In this instance, the second molded portion is formed with its front end tightly sealed to, and directly united with the rear end of the first molded portion. Therefore, even though the sheath and the sheds are molded as axially divided pieces, the adjacent molded portions are united with each other to provide an axially continuous sheath and an excellent insulating property of the composite insulator.
The process for manufacturing the composite insulators according to the applicant's earlier proposal proved to be highly advantageous in that elongate products with a high accuracy and an excellent insulating property can be produced in a facilitated and simplified manner, without increasing the length of the mold, under appropriately adjusted mold conditions. Still, it would be desirable to enable a more reliable manufacture of the composite insulator having a further improved product quality, while maintaining the advantage of the earlier proposal.